I don't know about the others, because I mainly do photo stuff ( Darktable seems decent though, and it has a been a long time since I've run linux on my desktop computer so I don't know how far it went ), but the GIMP is NOT functional in any way for a professional and a professional who is not willing to spend some dollars to get something that triples his productivity is NOT a professional.

The GIMP doesn't even support something like Adjustment Layers and that's a feature that was added to Photoshop 4.0 in 1996. Get real. Those who make that kind of argument for the GIMP, even pretending it could be used as a professional tool, have no idea what they're talking about.

Even the all-public, cheap edition of Photoshop, Photoshop Elements, that you can get for $60, has features like Adjustment Layers.

The GIMP has been working on that stuff for a long time and they're still not done yet. They put off all features related to non-destructive editing until they finally fully switch to the GEGL engine.